I have no idea on how to enable that.
I think it’s fairly reasonable to assume it is still not available to older SDKs until it is announced as available, if ever.
Too time to wait...
The 13.1 release compiler release announces itself as
5.5.1-dev, so most likely has been built from the 5.5 release branch (or something similar), I wouldn't expect something huge as the backport deployment to come up in a service release. I expect this to be 5.6 material, at the earliest.
No, Xcode 13.1 does not provide support for back-deployed concurrency.